Heating units of the type of the invention and/or the shield thereof must be protected against overheating above a limiting temperature, e.g., by a protective switch operating in a temperature-dependent manner, so as to prevent heating on the glass ceramic top above several 100.degree. C., e.g. approximately 600.degree. C. (EP 288 915=U.S. Pat. No. 4,845,340). As such plates have a very low thermal mass and are scarcely conductive with respect to metallic materials, e.g. steel, the temperature sensor for such protection must be positioned in such a way that heat transfer thereto takes place by direct radiation, back radiation and/or a fluid or air in a substantially tightly closed or unventilated area, through which the thermal radiation is directed towards the shield. In order to achieve this the temperature sensor is appropriately positioned at least partly in that area, namely, between the emission side of at least one heater and the bearing plane for the shield. Due to the low conductivity of the plate or the heating unit it has hitherto been assumed that the temperature sensor must extend over the entire area associated with said heating means, so as to ensure an effective protection also in the case of local, narrowly defined overheating. Thus, the temperature sensor generally forms a rod traversing the area and whose ends engage in boundary walls thereof. Thus, for each size of a heating unit or such an area a different temperature sensor is required, which is disadvantageous as regards manufacture, stockkeeping and installation. There are also spacers between the temperature sensor, support body and plate.
DE 35 36 981 discloses a radiant heating unit, in which the temperature sensor does not extend over the entire heating field. Its end is provided with a mounting support, which is supported on the insulation of the support body and/or on the plate to be heated. The mounting support is interconnected between the sleeve forming the temperature sensor and its inner rod and is fixed in the insulation.
Shorter temperature sensors only extending over a short distance into the area or over the heating means, can admittedly be used for the operation of signal devices for heat indication purposes, but they function in a temperature range well below 100.degree. C. (DE 26 27 373). Compared with an overheating protection, much lower demands are made thereon with respect to the thermal loading, the switching accuracy and the determination of the complete heating field. Thus, signal switches are not comparable with overheating switches, although the latter can additionally be constructed for the operation of a signal contact.
For an effective overheating protection, particularly of the shield or glass ceramic plate, it is also not possible to use temperature sensors located on the side of the heating means remote from the shield and which are, e.g., embedded in the insulating material support body (DE 36 22 415=U.S. 48 10 857). Thus, such temperature sensors are only suitable for monitoring a separately switchable marginal area of the heating means and for effective protection purposes it is necessary to have an additional temperature sensor located in said area.
In the case of mass hotplates protection can be provided by a temperature sensor, which is only located on one side of an axial plane of the cooking area positioned transversely thereto and on the side of the heating means remote from the heating or cooking surface, because in that location there is no need to fear local overheating due to the good thermal conductivity of the hotplate body (DE-U-76 12 737). This temperature sensor is also not heated to the same extent as the hotplate body and instead operates at a much lower temperature, which is derived from that of the hotplate body, so that it responds much more slowly than a temperature sensor in a radiant heating unit.
The radiant heating unit can have an overall heating field, which is subdivided into separately operable individual fields, so that through the choice of the number of simultaneously operated and interconnected individual fields it is possible to vary the operating size and shape of the heating field. The smallest operating shape or size, whose associated heating field must be put into operation in each operating state, must then have the said monitoring by the temperature sensor, so that the latter forms a thermally reactive working section with a longitudinal portion in the vicinity of said individual field. Longitudinal portions connecting thereto and possibly extending into adjacent heating fields must be thermally less reactive or non-reacting, in that they are insulated with respect to the radiation of the associated heating means, e.g. using at least one shield (DE 34 10 442=U.S. Pat. No. 4,633,238 and GB 22 29 615).
The invention solves the problems of the prior art by providing a radiant heating units of the aforementioned type, through which disadvantages of known constructions or of the described type are avoided and which, particularly with reliable overheating protection, ensures a simple construction and/or manufacture.